European Council President Donald Tusk tells reporters the case in which he was questioned for over eight hours as a witness in Warsaw, Poland, is of highly political character, Wednesday, April 19, 2017. The investigation deals with alleged secret illegal contacts between Polish and Russian intelligence officials at a time when Tusk was Poland's prime minister from 2007-2014. (AP Photo/Czarek Sokolowski)

EN_01254902_1886 AP

European Council President Donald Tusk tells reporters the case in which he was questioned for over eight hours as a witness in Warsaw, Poland, is of highly political character, Wednesday, April 19, 2017. The investigation deals with alleged secret illegal contacts between Polish and Russian intelligence officials at a time when Tusk was Poland's prime minister from 2007-2014. (AP Photo/Czarek Sokolowski), APTOPIX

EN_01254902_1887 AP

European Council President Donald Tusk tells reporters the case in which he was questioned for over eight hours as a witness in Warsaw, Poland, is of highly political character, Wednesday, April 19, 2017. The investigation deals with alleged secret illegal contacts between Polish and Russian intelligence officials at a time when Tusk was Poland's prime minister from 2007-2014. (AP Photo/Czarek Sokolowski)

EN_01254902_1888 AP

European Council President Donald Tusk tells reporters the case in which he was questioned for over eight hours as a witness in Warsaw, Poland, is of highly political character, Wednesday, April 19, 2017. The investigation deals with alleged secret illegal contacts between Polish and Russian intelligence officials at a time when Tusk was Poland's prime minister from 2007-2014. (AP Photo/Czarek Sokolowski)

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President of European Council and former Polish Prime Minister Donald Tusk is seen leaving the prosecutor's office where he testified for about 10 hours as a witness in the investigation on the cooperation of the Polish Military counter-intelligence Service with the Russian Federal Security Service in Warsaw, Poland on April 19, 2017. / AFP PHOTO / Wojtek RADWANSKI

EN_01254903_1962 AFP

President of the European Council and former Polish Prime Minister Donald Tusk is seen leaving the prosecutor's office where he testified for about 10 hours as a witness in the investigation on the cooperation of the Polish Military counter-intelligence Service with the Russian Federal Security Service in Warsaw, Poland on April 19, 2017. / AFP PHOTO / Wojtek RADWANSKI

EN_01254903_1964 AFP

President of the European Council and former Polish Prime Minister Donald Tusk is seen leaving the prosecutor's office where he testified for about 10 hours as a witness in the investigation on the cooperation of the Polish Military counter-intelligence Service with the Russian Federal Security Service in Warsaw, Poland on April 19, 2017. / AFP PHOTO / Wojtek RADWANSKI

EN_01254903_1965 AFP

President of the European Council and former Polish Prime Minister Donald Tusk is seen leaving the prosecutor's office where he testified for about 10 hours as a witness in the investigation on the cooperation of the Polish Military counter-intelligence Service with the Russian Federal Security Service in Warsaw, Poland on April 19, 2017. / AFP PHOTO / Wojtek RADWANSKI

EN_01254903_1966 AFP

President of the European Council and former Polish Prime Minister Donald Tusk is seen leaving the prosecutor's office where he testified for about 10 hours as a witness in the investigation on the cooperation of the Polish Military counter-intelligence Service with the Russian Federal Security Service in Warsaw, Poland on April 19, 2017. / AFP PHOTO / Wojtek RADWANSKI

EN_01254903_1972 AFP

President of the European Council and former Polish Prime Minister Donald Tusk is seen leaving the prosecutor's office where he testified for about 10 hours as a witness in the investigation on the cooperation of the Polish Military counter-intelligence Service with the Russian Federal Security Service in Warsaw, Poland on April 19, 2017. / AFP PHOTO / Wojtek RADWANSKI

EN_01254903_1973 AFP

President of the European Council and former Polish Prime Minister Donald Tusk is seen leaving the prosecutor's office where he testified for about 10 hours as a witness in the investigation on the cooperation of the Polish Military counter-intelligence Service with the Russian Federal Security Service in Warsaw, Poland on April 19, 2017. / AFP PHOTO / Wojtek RADWANSKI

EN_01254903_1974 AFP

President of the European Council and former Polish Prime Minister Donald Tusk is seen leaving the prosecutor's office where he testified for about 10 hours as a witness in the investigation on the cooperation of the Polish Military counter-intelligence Service with the Russian Federal Security Service in Warsaw, Poland on April 19, 2017. / AFP PHOTO / Wojtek RADWANSKI